Signals used in mobile communication devices, such as a cell phone, base station apparatuses for them, network infrastructure devices, such as a server and a router, large-sized computers, and the like are being increased in the speed and capacity. In accordance with this, printed wiring boards mounted on these electronic devices are required to adapt to high-frequency signals, and a substrate material having excellent dielectric characteristics (low dielectric constant and low dielectric dissipation factor; hereinafter these may be referred to as high frequency properties) in a high frequency range that enable reduction of a transmission loss is demanded. Recently, as such applications that handle high-frequency signals, practical realization and practical use planning of novel systems that handle high-frequency wireless signals in an ITS field (in connection with automobiles and traffic systems) as well as in a field of indoor short-distance communications systems, in addition to the above-mentioned electronic devices, is being promoted, and in future, low transmission-loss substrate materials are expected to be required for the printed wiring boards to be mounted on these devices.
Further, in view of the environmental problems encountered in recent years, mounting of electronic parts using a lead-free solder and achieving flame retardancy free of a halogen are demanded, and therefore the material for a printed wiring board is needed to have higher heat resistance and more excellent flame retardancy than conventional.
Conventionally, for a printed wiring board required to have a low transmission loss, a polyphenylene ether (PPE) resin has been used as a heat-resistant thermoplastic polymer excellent in high frequency properties. For example, a method of using a polyphenylene ether and a thermosetting resin as combined has been proposed. Specifically, a resin composition containing a polyphenylene ether and an epoxy resin (see, for example, PTL 1), a resin composition using a polyphenylene ether in connection with a cyanate ester resin having a low dielectric constant among thermosetting resins (see, for example, PTL 2) and the like have been disclosed.
However, the resin compositions described in the above-mentioned PTL's 1 and 2 are unsatisfactory collectively in the high frequency properties in a GHz region, the adhesion to conductor, the low thermal expansion coefficient, and the flame retardancy. In addition, the compatibility of polyphenylene ether with a thermosetting resin is low, and therefore the heat resistance of the resultant resin composition may often lower.
Meanwhile, the present inventors have proposed a resin composition having a polyphenylene ether resin and a polybutadiene resin as a base, wherein the resin composition can be improved in the compatibility, heat resistance, low thermal expansion coefficient, adhesion to conductor, and the like by performing semi-IPN (semi-interpenetrating network) formation in the production stage (A-stage) of producing a resin composition containing an organic solvent (for example, PTL 3). However, the substrate material recently used for a printed wiring board is not only required to adapt to high-frequency signals, but also required to have high adhesion to conductor, low thermal expansion coefficient, high glass transition temperature, high flame retardancy, and the like due to the demands for an increase in the density, high reliability, and adaptability to consideration of the environment.
For example, the adhesion to conductor is desired to be 0.58 kN/m or more, and further 0.6 kN/m or more, in terms of a copper foil peel strength as measured using a low profile copper foil (Rz: 1 to 2 μm) having a very small surface roughness on the side to be bonded to resin.
Further, the substrate material for printed wiring board used in the application of network related devices, such as a server and a router, is needed to be stacked into the increased number of layers as the density of the wiring board is increased, and therefore the substrate material is required to have high reflow heat resistance and through-hole reliability. The glass transition temperature of the material as a yardstick for the above properties is desirably 200° C. or higher, and the thermal expansion coefficient (in the Z-direction at the Tg or lower) is desirably 45 ppm/° C. or less, further desirably 43 ppm/° C. or less. For achieving low thermal expansion property, the incorporation of an inorganic filler into the resin composition is effective; however, in a multilayer printed wiring board with the increased number of layers, for surely obtaining the flow properties of the resin for circuit packing, the amount of the inorganic filler to be incorporated is restricted. Therefore, it is desired that even when the amount of the inorganic filler incorporated is relatively small, the resultant resin composition is desired to secure the above-mentioned required values.
With respect to high frequency properties, excellent dielectric properties in a higher frequency range are desired, and a substrate material using an ordinary E glass substrate is desired to have a dielectric constant of 3.8 or less, more desirably 3.7 or less, and even more desirably 3.6 or less, and to have an dielectric dissipation factor of 0.007 or less, more desirably 0.006 or less. Furthermore, a substrate material generally tends to have an increased dielectric dissipation factor at a higher frequency, but is increasingly strongly needed to satisfy the above-mentioned required values in a 10 GHz or more band which is a frequency band higher than the conventional 1 to 5 GHz band for the dielectric properties values.
For securing flame retardancy, in general, a halogen element-containing flame retardant, especially a bromine-based flame retardant has been used. However, from the viewpoint of recent global environment conservation and aggravation prevention, a technique for flame retardation not using a halogen element (especially chlorine atom, bromine atom) that has a risk of generation of dioxins, benzofuran or the like has become desired.